1. Field of the Invention
This invention relates to a turbocharger applied to a car engine.
2. Description of the Prior Art
A turbocharger which can provide supercharging of a suitable and sufficient quantity of air in a wide range of a low speed to a high speed in an internal combustion engine equipped with a turbocharger has been disclosed, for example, in Japanese Patent Laid-Open No. 195329/1985 filed by the inventor of the present invention. The turbocharger of this prior art will be described with reference to FIG. 4 of the accompanying drawings. The turbocharger 40 of the internal combustion engine includes a turbine 41 which is driven by the exhaust gas energy of the internal combustion engine and a compressor 42 which supercharges air into a cylinder when driven by the turbine 41, and further includes a motor-generator 43 which is driven by the turbine 41. The turbine 41, the motor-generator 43 and the compressor 42 are disposed sequentially on a shaft 44. This shaft 44 is fitted rotatably to a housing 48 through a bearing 49 and a floating metal 50. The motor-generator 43 consists of a magnetic rotor 46 fixed to the shaft 44 and a stator coil 47 fitted to the housing 48 and is operated as a motor or a generator in accordance with the operating condition of the internal combustion engine. The motor-generator 43 is operated as the motor in at least the low speed/high load operating range of the internal combustion engine.
In the turbocharger 40 of the internal combustion engine described above, the turbine 41 receives the heat energy from the exhaust gas energy and attains a high temperature. However, since the A.C. generator-motor, that is, the motor-generator 43 equipped with the magnetic rotor 46 consisting of a permanent magnet, is disposed on the shaft 44 that connects and drives the turbine 41 and the compressor 42, the magnetic rotor 46 disposed between the turbine 41 and the compressor 42 is affected by the heat of the turbine 41 or the heat transferred thereto by heat transfer and radiation through the turbine blade 45 and the shaft 44 when the turbine 41 reaches a high temperature. Here, the problem develops in that the magnetic rotor 46 made of the permanent magnet in the motor-generator 43 is not much resistant to the heat of high temperature. In other words, the permanent magnet is very weak to the heat and when the temperature is about 200.degree. C. or more, its demagnetizing factor gets deteriorated drastically. For example, the demagnetizing factor is about 5% at about 200.degree. C. but is as high as about 30% at about 300.degree. C. If the turbocharger is stopped rapidly particularly when it is operated at a high load, the overall temperature becomes so high that the A.C. magnetic rotor as the permanent magnet is demagnetized disadvantageously. For instance, there is the possibility that the permanent magnet gets demagnetized by the heat of the exhaust system of the engine when the engine stops after the car drives on a highway and comes to halt, for example.
Furthermore, a lubricating oil for lubricating the bearings 49 and the floating metal 50 that support the shaft of the turbocharger 40 is carbonized and adheres by the heat from the turbine 41 receiving the exhaust gas energy and this adhering lubricating oil brings the bearings 49 and the floating metal 50 into a sticky state. Accordingly, when the turbocharger is operated next, its operation is inhibited. Therefore, in the turbocharger equipped with the A.C. generator-motor in an internal combustion engine, there remains the problem how the bearings supporting the magnetic rotor of the A.C. generator-motor and the shaft can be protected from the heat transferred from the turbine.